Li-ion batteries are known to have significant heating during charging and discharging. At normal 1 C rate or lower, heating is not significant and no cooling is required. At higher C-rates, such as 5 C, 10 C and higher as demanded in automotive and other applications, the cell temperature will increase dramatically and additional cooling is required. In order to cool down the core temperature to keep the Li-ion cells within the optimum operating temperatures of 20-40° C., convective natural cooling, forced air cooling, and forced air or liquid cooling through conducting plates have been utilized.
All of these state-of-the-art cooling methods utilize the large surface areas of the flat (prismatic) battery design and try to cool the battery from the surface. However, the battery is built by multiple layers of cathode/separator/anode as a jelly-roll or in stacks. Heat conduction through these layers is the least preferred direction because of the thermal resistance between the layers and the low thermal conductivity of the separators, which are typically polymeric membranes. Although copper and aluminum are used as current collectors, the effective thermal conductivity through the layers is about 1-2 W/mK as the effective conductance is the series sum of the conductivities. Cooling from the flat surfaces is very ineffective and usually the middle layer temperature can keep increasing during high C-rate discharge. These conditions have limited current Li-ion battery designs to be large in surface area (similar to 11″×8.5″ paper) and small in thickness (6-8 mm) and 4 C-5 C maximum discharge rate. However, if the thickness of the batteries could be increased without increasing the surface area, for example high capacity cells could be produced with only 50 cells for a battery of 24 kWh. This would reduce the cost of the overall battery pack because of easier battery management and significant reduction of the peripherals related to connections and system complexity. In addition, more compact packing of the cells would be possible, thus increasing overall energy density of the battery pack.